This study is designed to clarify the reproductive neuroendocrinology of melatonin action by localizing and characterizing its binding in hamster brain. This pineal hormone is responsible for the seasonal modulation of reproduction by daylength in a variety of species including the hamster. Putative receptors for melatonin will be localized and characterized by the following procedures: 1) Autoradiographic analysis of melatonin uptake through In Vivo and In Vitro techniques. This approach will allow as fine a neuroanatomic localization as is currently possible. 2) In Vitro binding studies of specific brain regions and nuclei. This will allow determination of the intracellular distribution of melatonin in target tissues. 3) Studies of the consequences of manipulating photoperiod and reproductive state on melatonin uptake and binding characteristics. This will implicate putative melatonin receptors in physiological responses. Experiments of these types are necessary for future work on mechanisms of photoperiodic modulation of reproduction. These experiments will ultimately provide better understanding of environmental influences on brain-pituitary-gonadal interrelationships. They will exploit the advantages of seasonal breeding as a model of reversible fertility and may lead to improvements in food production and human contraception. Finally, study of the neuroendocrine mechanisms by which daylength drives annual breeding cycles may lead to a better understanding of brain plasticity.